Dimmer circuits are used to control the power provided to a load such as a light or electric motor from a power source such as mains. Such circuits often use a technique referred to as phase controlled dimming. This allows power provided to the load to be controlled by varying the amount of time that a switch connecting the load to the power source is conducting during a given cycle.
For example, if voltage provided by the power source can be represented by a sine wave, then magnum power is provided to the load if the switch connecting the load to the power source is on at all times. In this way the the total energy of the power source is transferred to the load. If the switch is turned off for a portion of each cycle (both positive and negative), then a proportional amount of the sine wave is effectively isolated from the load, thus reducing the average energy provided to the load. For example, if the switch is turned on and off half way through each cycle, then only half of the power will be transferred to the load. Because these types of circuits are often used with resistive loads and not inductive loads, the effect of repeatedly switching on and off power will not be noticeable as the resistive load has an inherent inertia to it. The overall effect will be, for example in the case of a light, a smooth dimming action resulting in the control of the luminosity of the light. This technique will be well understood by the person skilled in the art.
A technique commonly referred to as mains ripple injection is used as a means of simple on-off enable control of mains powered equipment such as household hot water heaters. A signal in the form of a low frequency sinewave of several hundred Hertz or more, with magnitude typically of around ten volts r.m.s., is superimposed on the ac voltage waveform of the mains supply itself. There is no fixed phase relationship between the injected ripple frequency and the mains supply frequency ie. a beat frequency component results.
In many dimmer circuit designs the presence of mains ripple injection results in undesirable lamp intensity flickering. This effect is primarily due to variation in dimmer conduction angle, corresponding to beating between ripple signal frequency and mains frequency.
Typical dimmer designs utilize mains zero crossing detection for control circuit synchronization and therefore can suffer from conduction angle timing variations due to ripple signals. More elaborate designs will incorporate the necessary filtering elements to attenuate such ripple induced variations.
It is therefore an object of the present invention to provide an effective means of reducing the effects of mains ripple injection in a dimmer circuit.